Mobile cellular communication is evolving beyond traditional voice telephony towards more sophisticated services, such as Push-To-Talk (PTT). Similar to conventional walkie-talkie communication, PTT enables mobile communication users to send a voice message to one or more recipients over a mobile phone by simply pushing a key (i.e., PTT button, etc.).
One particular version of PTT, called PoC (PTT-over-Cellular), has started to be implemented in wireless data networks such as GSM/GPRS and CDMA cellular networks. By using internet protocols (i.e., an internet protocol network), these networks can provide a packet-based data service that enables information to be sent and received across a mobile telephone network. In addition, the use of internet protocols also facilitates PoC through the use of instant connections. That is, information can be sent or received immediately as the need arises, subject to available time slots at the air interface.
PTT, including PoC-based PTT, is half-duplex. That is, all participants typically use a single frequency or channel for both transmission and reception. Either a participant speaks or listens, but not both. This is in contrast to traditional cellular communication that is full-duplex (e.g., like a regular wired phone), in which at least one channel or frequency is assigned to talk, and another separate one is assigned to listen such that both speaking and listening can occur simultaneously.
For audio/video data transmissions, PoC applications require the transmission of signaling packets using a signaling protocol, e.g., SIP (Session Initiation Protocol), and data packets using a data protocol, e.g., RTP (Real Time Protocol). SIP is a signaling protocol for Internet conferencing, telephony, presence, events notification, and instant messaging. RTP is an Internet-standard protocol for the transport of real-time data, including audio and video media. It can be used for media-on-demand as well as interactive services such as Internet telephony. RTP consists of a data and a control part. The latter is called RTCP.
PoC is discussed in greater detail in the following technical specifications which are incorporated by reference: Push-to-talk over Cellular (PoC), Architecture, PoC Release 2.0, V2.0.8 (2004–06); Push-to-talk over Cellular (PoC), Signaling Flows—UE to Network Interface (UNI), PoC Release 2.0, V2.0.6 (2004–06); and Push-to-talk over Cellular (PoC) User Plane, Transport Protocols, PoC Release 2.0, V2.0.8 (2004–06). Of note, Release 1.0 is also available from the PoC Consortium as well as an upcoming PoC standard from Open Mobile Alliance (OMA). All of these are generally considered native PoC standards. Subsequently, UE (user equipment), such as a PoC enabled cellular phone, supporting either of these standards is called a native PoC client. Additional information is found in IETF RFC 3261, which is incorporated herein by reference. This document describes Session Initiation Protocol (SIP), which is an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences.